Distinct from field effect transistors (FETs), electrons and holes may be current carriers in bipolar junction transistors (BJTS). Since the bipolar junction transistors may have fast operational speeds, bipolar junction transistors may be used in high-speed and high-frequency devices.
FIG. 1 illustrates a structure of a related art bipolar junction transistor.
Referring to FIG. 1, a high-density N type buried layer 12 may be formed on a semiconductor substrate, and formed with an isolation layer. N type well area 13 and high-density N type plug area 14 may be formed on the resultant structure. Low-density P type base area 15 may be formed in well area 13, and high-density N type emitter area 16 and high-density P type base area 17 may be formed in base area 15. Emitter area 16 and base area 17 may be connected to emitter layer 18 and base layer 19, which include polysilicon, and may be formed on the emitter area 16 and the base area 17, respectively. High-density N type collector area 20 may be formed in plug area 14. Emitter layer 18, base layer 19, and collector area 20 may be covered with inter-layer dielectric layer 21 and connected to contacts 22 formed in inter-layer dielectric layer 21, respectively.
To obtain a high-speed bipolar junction transistor, a base width (WB) thereof may be formed as narrowly as possible. However, since a related art bipolar junction transistor may have a planar type structure, it may be difficult to narrow the base width (WB). In addition, since base layer 19 and emitter layer 18 may be stacked to reduce base resistance, a manufacturing process may be complicated.